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DOI: 10.1039/C5RA12055A (Paper) RSC Adv., 2015, 5, 58416-58427

Potentiometric multi-walled carbon nanotube Zn-sensor based on a naphthalocyanine neutral carrier: experimental and theoretical studies

Ola R. Shehab* and Ahmed M. Mansour
Chemistry Department, Faculty of Science, Cairo University, Gamaa Street, Giza 12613, Egypt. E-mail: olashehab_chem@yahoo.com; Fax: +20 2 35728843; Tel: +20 2 01001905350

Received 23rd June 2015 , Accepted 26th June 2015

First published on 26th June 2015

Abstract

A new multi-walled carbon nanotube graphite paste sensor based on 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine as a neutral carrier (2.0%), 2-flouorophenyl-2-nitrophenyl ether (50.0%) as a plasticizer, and sodium tetrakis-imidazolyl borate (1.0%) as an anionic additive has been explored as a selective sensor for determination of Zn2+ in real samples. The electrode showed a fast response time of 5 s, gave a Nernstian response (29.9 mV per decade) over the concentration range 1.0 × 10−8 to 1.5 × 10−4 mol L−1, and could be used in the pH range of 4.3–7.5 with a detection limit of 5.0 × 10−9 mol L−1. The response mechanism of the electrode was investigated using UV-vis and FT IR spectroscopy. Scanning electron microscopy combined with energy dispersive X-ray spectra were used to confirm the reaction between Zn2+ ions and naphthalocyanine on the surface of the electrode. In order to predict the selectivity of the naphthalocyanine sensor for different metal ions, the corresponding binding energies of the metal complexes were calculated at the Hartree–Fock level of theory.

1. Introduction

Chemically modified carbon paste electrodes (CMCPEs) have been successfully applied as potentiometric sensors for the determination of various species.1–4 Compared to the other types of ion selective electrodes (ISEs), CMCPEs are extremely simple to prepare, easy to regenerate, and give a stable response with a very low ohmic resistance,5,6 corresponding to the formation of a very thin film of a pasting liquid coated onto small particles of carbon powder.7 Multi-walled carbon nanotubes (MWCNTs) were discovered by Iijima,8 which have attracted great interest owing to their highly accessible surface area with a narrow size distribution, excellent electrical conductivity, and high stability. These properties encouraged analysts for using MWCNTs in the composition of sensors instead of normal graphite with a large particle size. The electrodes contained MWCNTs showed excellent features such as high sensitivity, low detection limit, and fast response that may be attributed to the signal enhancement provided by high surface area, low overvoltage, and rapid electrode kinetics.9 Moreover, an improvement in the electric conductivity and mechanical properties of the paste matrix was reported.

In the present study, the choice of Zn2+ is ascribed for the presence of Zn2+ as a component of several enzymes such as carboanhydrases, and proteinases.10–12 It is the second most abundant transition metal ion in the human body and plays significant role in many biological processes such as brain function and pathology, immune function, gene transcription. In addition, Zn(II) has unique ability to promote rewinding of the melted DNA, and can stimulate hydrolysis of DNA and RNA.13 Zinc and its compounds are widely used in electroplating, paint, rubber, dye, wood preservatives, and batteries. However, its presence in high protein foods and its large doses can cause fever, chills, pulmonary manifestation, gastroenteritis, vomiting, nausea, anemia and renal failure. Besides, a major problem is the subsequent pollution and harm to the environment and humans because of its frequent use.14 For example, common zinc compounds such as zinc chloride, zinc oxide, zinc sulfate and zinc sulfide are found at hazardous waste sites. In view of its toxicity, the determination of Zn2+ becomes more important. Several analytical techniques have been used for determination of Zn2+ such as flame atomic absorption spectrometry,15 fluorimetry,16 UV-vis,17 ICP-AES,18 potentiometry,19–25 and voltammetry.26

Macrocycles are most likely to be used in ISEs, however not all macrocycles are useful as sensing electrodes. They should provide high complexation or extraction selectivity for a particular metal ion and enough conformational flexibility for fast ion exchange. Besides that they must have high lipophilicity to remain in membrane and moderate molecular weight to allow high mobility. The function and applications of metallonaphthalocyanine coordination compounds are exciting as it expands from them being catalytic centers, through photosensitizing agents in therapy and other applications, to key cofactors in several biological systems. The selected macrocyclic ligand in this study (naphthalocyanine motif) has attracting interest in the field of construction and design of new sensor capable of detecting definite metal ion of compatible dimensions in his electron rich heart cavity.27,28 Based on various parameters, the high stability and selectivity of its metal ion complex, its solubility and high ability to extract the metal ion into the paste phase, 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine was explored as an active material in carbon paste electrode for the fabrication of Zn2+ selective sensor. Comparison between the proposed electrode and the reported Zn2+ sensors based on macrocyclic ionophores27–35 showed that the developed sensor has superiority over the reported electrodes in terms of low detection limit and fast response time (Table 1).

Table 1 Comparison of the proposed Zn-MWCNTCP electrode with the reported Zn2+ sensors based on macrocyclic ionophores
Ion-recognition Slope, mV Linear range, M LOD, M Response time, s Ref.
5,10,15,20-Tertraphenyl-21H,23H-porphine 29.0 6.2 × 10−6 to 1.0 × 10−1 — 12 31
Tetramethyl-8,13-divinyl-2,18-porphine-dipropionic acid 30.0 1.3 × 10−5 to 1.0 × 10−1 — 10 30
Hematoporphyrin IX 28.6 5.0 × 10−5 to 1.0 × 10−1 — 30 33
Protoporphyrin IX 29.0 1.5 × 10−5 to 1.0 × 10−1 —   32
Benzo-substituted macrocyclic diamide 30.0 9.0 × 10−5 to 1.0 × 10−1 5.0 × 10−5 20 34
Bzo2Me2Ph2(16)hexaeneN4 28.5 2.8 × 10−6 to 1.0 × 10−1 2.2 × 10−6 <10 35
12-Crown-4 29.5 7.0 × 10−5 to 1.0 × 10−1 — 10 27
Triaza-5,8-dioxo-3(4),9(10)-dibenzoyl-1,12,14-triene 29.2 1.3 × 10−7 to 1.0 × 10−1 1.0 × 10−8 7 29
Thiazolidin-4-one 29.0 9.2 × 10−5 to 1.0 × 10−2 2.0 × 10−7 12 28
Tetra-tert-butyl-2,3-naphthalocyanine 29.9 1.0 × 10−8 to 1.5 × 10−4 5.0 × 10−9 5 This work

2. Experimental

2.1. Chemicals

Graphite powder, 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine, sodium tetraphenylborate (NaTPB), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), sodium tetrakis imidazolyl borate (NaTImB), dibutyl-butyl phosphonate (DBBP), o-nitrophenyloctyl ether (o-NPOE), dioctyl adipate (DOA), tricresyl phosphate (TCP), 2-flouorophenyl-2-nitrophenyl ether (FPNPE), and THF were purchased from Aldrich chemical company. MWCNT (3–20 nm OD, 1–3 nm ID, 0.1–10 micro long) was purchased from Alfa Aesar. NaNO3, KNO3, NH4NO3, Cd(NO3)2, Ba(NO3)2, Ni(NO3)2·6H2O, Pb(NO3)2, Mg(NO3)2·6H2O, Mn(NO3)2·4H2O, Al(NO3)3·9H2O, Cu(NO3)2·3H2O, and Ca(NO3)2·4H2O were purchased from Oxford, India.

2.2. Apparatus

Digital Jenway 3010, and 3505 pH meters were used for potential, pH measurements, respectively. A SENTEK R1/2MM Ag/AgCl electrode was used as the outer reference electrode. UV-vis spectra were recorded on a Shimdazu Lambda 4B spectrophotometer. FT IR-460 plus JASCO 4000–400 cm−1 was used for IR measurements (KBr pellets & solutions in THF). Perkin-Elmer Optima 2000 ICP instrument was used for the atomic emission spectrometry measurements. Quanta FEG 250 instrument was used to obtain the scanning electron micrographs (SEM) of the electrodes.

2.3. Sensors preparation

The modified paste was prepared by mixing the appropriate weight of naphthalocyanine, and highly pure graphite with acetone. The mixture is homogenized by careful mixing with agate pestle in agate mortar, left at room temperature to evaporate acetone, and then a weighed amount of the plasticizer is added. Similar, the MWCNT sensor was prepared by the same way with the addition of the MWCNT to the graphite powder. The paste is then packed into the electrode body.36 All EMF measurements were carried out with the following cell assembly:

Ag/AgCl, KCl (3 M)/test solution/filling graphite modified paste/carbon paste electrode.

2.4. Selectivity of the sensor

Potentiometric selectivity factor (KPotA,B) was evaluated using the separate solution method (SSM)37 according to the following relation:
image file: c5ra12055a-t1.tif
where EA, EB are the electrode potentials of 1.0 × 10−3 mol L−1 solution of each of Zn2+ and interfering cation, ZA, ZB are the charges on Zn2+ and the interfering cations, respectively and S is the theoretical slope.

2.5. Potentiometric determination

Standard addition method was applied for the potentiometric determination.38 In this method, known amounts of standard Zn2+ were added to a sample of 25.0 mL with the unknown concentration and from the potential change, ΔE = (Eu − Es) one can determine the concentration of the test sample using the equation:
image file: c5ra12055a-t2.tif
where Cx is the unknown concentration, Vx is the volume of sample solution, Vs and Cs are the volume and concentration of the standard Zn2+ solution added to the sample, respectively, ΔE is the change in potential after addition of certain volume of standard solution, and S is the slope of the calibration graph.

2.6. Computational details

Full-unconstrained geometry optimization of 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine and its complexes have been carried out at Hartree–Fock (HF) level of theory. In the model structures of naphthalocyanine, the naphthalene and tertiary butyl groups were replaced by H atoms36 to reduce the computer time. The standard 6-31G(d) basis set was used for Na+, Mg2+, Al3+, K+, Ca2+, Mn2+, Ni2+, Cu2+, and Zn2+, while the effective-core potential (ECP) of LANL2DZ was applied for Cd2+, Ba2+ and Pb2+. The effect of solvent on the theoretical parameters was performed using the default Polarized Continuum Model (PCM),39 where water as a solvent was considered as a uniform medium (ε = 78.39). Ionization energy, electron affinity, hardness, softness, dipole moment, and energy gap were calculated according to Koopman's theorem.40 Natural bonding orbitals (NBO) analysis41 of Mn2+, Ni2+, Cu2+, and Zn2+ complexes were performed using NBO 3.1 program at the HF/6-31G(d) level of theory. All the calculations were carried out by Gaussian 03 package.42

3. Results and discussion

3.1. Factors affecting the performance of the Zn2+ sensor

3.1.1. Effect of composition. It is well known that the performance characteristics (sensitivity, selectively, detection limit, and linear range) depend on the properties of the plasticizer, graphite/plasticizer ratio, and the nature and the amount of the ionophore.43 In the present study, the influence of the amount of naphthalocyanine as an ionophore on the potential response of the developed CMCP electrode as a function of the zinc activity was investigated in the concentration range from 1.0 × 10−8 to 1.0 × 10−3 mol L−1. As shown in Table 2, the electrodes containing only 2 and 3% of the ionophore exhibited super Nernstian response towards Zn2+ ions with slopes of 43.0, 55.0 mV per concentration decade, narrow linear ranges of 2.5 × 10−4 to 5.0 × 10−3, 3.2 × 10−4 to 3.2 × 10−3 mol L−1 and high detection limits of 1.6 × 10−4, 1.0 × 10−4 mol L−1, respectively. To enhance the performance characteristics of the present sensors, other paste components such as cationic/anionic additive, type and amount of plasticizers, etc. needed to incorporate and study.44–46 It was found that the electrode dipped with NaTFPB as anionic additive showed better performance in terms of slope, detection limit, and linear range. For example, the slope of the electrode no. 4 comes closer to the Nernstian (33.2 mV) with a wider linear range of 3.9 × 10−6 to 5.0 × 10−3 mol L−1 and the detection limit is decreased to 3.8 × 10−6 mol L−1 comparing with the non-dipped one. While the incorporation of CTAB as cationic additive into the modified paste diminishes the slope of the sensor which makes it not applicable. Then, the effect of the amount of naphthalocyanine was studied in presence of NaTFPB as anionic additive aiming to obtain superior performance. The results (Table 2) indicated that the optimum behavior was obtained by using 2% (w/w) of the ionophore. Further increase in the amount of the ionophore led to an increase in the slope and the detection limit of the sensor. By using the same mass percentage of the ionophore and changing the type of the additive from NaTFPB to NaTPB or NaTImB, it can be seen that the electrode dipped with NaTImB (Table 2, no. 6) gave better Nernstian response (29.9 mV) and detection limit of 2.13 × 10−6 mol L−1 comparing with the other studied anionic additives.
Table 2 Composition and slope of calibration curves for Zn2+-CMCPEsa
No. %composition Slope (mV) linear range (M) Detection limit (M) Relative standard deviationb%
Ionophore Graphite Plasticizer Additive
a M: multiwalled carbon nanotube.b Average of four replicates.
1 2 49.0 49.0NPOE — 43.0 2.5 × 10−4 to 5.0 × 10−3 1.60 × 10−4 1.12
2 3 49.0 48.0NPOE — 55.0 3.2 × 10−4 to 3.2 × 10−3 1.00 × 10−4 1.21
3 1 49.5 49.0NPOE 0.5NaTFPB 37.1 7.9 × 10−6 to 3.2 × 10−3 7.90 × 10−6 1.45
4 2 48.5 49.0NPOE 0.5NaTFPB 33.2 3.9 × 10−6 to 5.0 × 10−3 3.80 × 10−6 0.24
5 3 48.5 47.5NPOE 0.75NaTFPB 50.5 1.9 × 10−5 to 2.5 × 10−3 9.50 × 10−6 0.34
6 2 48.5 49.0NPOE 0.5NaTImB 29.9 3.9 × 10−6 to 3.9 × 10−3 2.13 × 10−6 0.33
7 2 48.5 49.0NPOE 0.5NaTPB 16.7 1.3 × 10−6 to 5.0 × 10−3 1.09 × 10−6 2.13
8 2 48.5 49.0DBP 0.5NaTImB 17.6 7.9 × 10−6 to 5.0 × 10−3 5.88 × 10−6 2.56
9 2 48.5 49.0DOA 0.5NaTImB 39.7 5.0 × 10−5 to 1.0 × 10−2 4.16 × 10−5 2.44
10 2 48.5 49.0TCP 0.5NaTImB 24.7 1.3 × 10−6 to 5.0 × 10−4 1.25 × 10−6 1.22
11 2 48.5 49.0DBBP 0.5NaTImB 55.7 1.6 × 10−4 to 5.0 × 10−3 7.94 × 10−5 1.78
12 2 48.5 49.0FPNPE 0.5NaTImB 26.6 3.9 × 10−7 to 5.0 × 10−3 3.98 × 10−7 0.66
13 2 48.0 49.0FPNPE 1.0NaTImB 29.3 2.0 × 10−7 to 1.5 × 10−3 2.00 × 10−7 0.44
14 2 47.0 50.0FPNPE 1.0NaTImB 28.7 3.9 × 10−7 to 9.3 × 10−4 3.90 × 10−7 0.33
15 2 46.5 + 0.5 M 50.0FPNPE 1.0NaTImB 29.5 5.0 × 10−8 to 1.3 × 10−5 5.01 × 10−8 0.21
16 2 46.0 + 1.0 M 50.0FPNPE 1.0NaTImB 29.9 1.0 × 10−8 to 1.5 × 10−4 5.01 × 10−9 0.23
17 2 45.5 + 1.5 M 50.0FPNPE 1.0NaTImB 29.5 5.0 × 10−8 to 1.5 × 10−4 1.00 × 10−8 0.36

Since the use of plasticizers gives some permeable properties to the paste and may improve the mechanical stability of the sensor,47 so it was necessary to study the response characteristics of the electrode in terms of the nature of plasticizer.43,48,49 For this, different types of plasticizers, NPOE (ε = 23.6), DBP (ε = 6.4), DOA (ε = 3.9), TCP (ε = 6.9), DBBP (ε = 4.6), and FPNPE (ε = 50.0) were examined. As shown in Table 2, the best performance was observed for the electrodes plasticized with NPOE and FPNPE with slopes 29.9, and 29.3 mV, in that order. The highest dielectric constant of FPNPE may be the main reason for decreasing the detection limit of the investigated electrode to 2.0 × 10−7 mol L−1. Therefore, FPNPE was chosen as the best plasticizer for the working electrode. Alternatively, highly accessible surface area with a narrow distribution size, excellent electrical conductivity, and high stability of MWCNT attract the authors to explore the performance of the naphthalocyanine electrode in terms of different amount of the added MWCNT. As shown in Table 2 (no. 16) and represented in Fig. 1, the electrode doped with 1% MWCNT showed great enhanced sensitivity observed by the lower detection limit, 5.0 × 10−9 mol L−1 compared with its analogue without MWCNT (no. 14).


image file: c5ra12055a-f1.tif
Fig. 1 Potentiometric response of Zn2+-CMCPE without MWCNT and with 1% MWCNT.
3.1.2. Effect of pH. In order to examine the pH effect on the potential response of the electrode, the potential values for the Zn2+ solutions containing concentrations of 10−2 to 10−4 mol L−1 and having different pH values were measured. The pH value from 1.0 to 10.0 was adjusted by adding diluted solutions of HNO3 or NaOH. The potential change as a function of pH is plotted in Fig. 2. The obtained data showed that the working pH range is 4.3–7.5. In alkaline medium, the possibility for the formation of Zn(OH)2 is present and below pH = 4.3, the change in the potential may be due to the protonation of the naphthalocyanine molecule, which will decrease its tendency to complex formation.
image file: c5ra12055a-f2.tif
Fig. 2 Effect of pH on the response of Zn2+-CMCP electrode based on 2% naphthalocyanine, 1% NaTImB, 48% graphite, and 49% FPNPE.
3.1.3. Response time. The response time of the electrodes was measured after the successive immersion of the electrodes in a series of Zn2+ solutions. In each solution, the Zn2+ concentration increased tenfold from 1.0 × 10−7 to 1.0 × 10−3 mol L−1 and 1.0 × 10−8 to 1.0 × 10−4 mol L−1 for electrodes no. 14 (without 1% MWCNT) and 16 (with 1% MWCNT). Comparison between the two pastes (Fig. 3) revealed that sensor no. 16 reaches the equilibrium potential (5 s) faster than the other (11 s). This indicates a rapid diffusion achievement of the equilibrium between the aqueous layer and the carbon paste phase, rapid complex formation, and exchange of ions.
image file: c5ra12055a-f3.tif
Fig. 3 Response time curves for the optimized Zn2+-CMPC electrode without and with 1% MWCNT.
3.1.4. Effect of temperature. The thermal stability of the sensors, calibration graphs (electrode potential vs. pZn2+) were measured in the temperature ranges of 25–50 °C (Table 3). The isothermal coefficient (dE°cell/dt) was determined from the linear relation between E°cell and temperature (t-25) as a slope (Fig. 4), where t is the experimental temperature.50 The values of the standard cell potential E°cell were obtained from the calibration plots as the intercept of these plots at pZn2+ = 0, and then were added to the standard electrode potential of the Ag/AgCl reference electrode at different temperatures to calculate the standard electrode (Zn-CMCPE) potentials (E°elec). The values of dE°cell/dt were found to be 2.11 × 10−3, and 4.66 × 10−4 V °C−1, while the values of dE°elec/dt were 1.345 × 10−3, and 1.23 × 10−3 V °C−1 for the sensors without and with 1% MWCNT, respectively. The obtained results exhibited fairly high thermal stability of the studied sensors within the investigated temperature range with no deviation from the Nernstian behavior.
Table 3 Performance characteristics of Zn2+-CMCPE at different test solution temperatures
Temperature (°C) Slope (mV per decade) Linear range (M)
0% MWCNT 1% MWCNT 0% MWCNT 1% MWCNT
25 29.96 29.50 3.98 × 10−7 to 1.28 × 10−2 1.00 × 10−8 to 1.47 × 10−4
30 30.95 29.98 4.57 × 10−7 to 5.49 × 10−2 1.00 × 10−8 to 1.50 × 10−4
40 32.79 34.76 3.98 × 10−7 to 1.14 × 10−2 1.00 × 10−8 to 1.00 × 10−4
50 33.10 35.85 3.98 × 10−7 to 3.54 × 10−2 1.00 × 10−8 to 1.11 × 10−4


image file: c5ra12055a-f4.tif
Fig. 4 Effect of temperature on standard cell potential and standard electrode potential for the Zn2+-CMCPE (a) 0% MWCNT and (b), 1% MWCNT.
3.1.5. Selectivity of electrodes. The selectivity coefficient is the main source of information concerning interferences on the electrode response and is defined by its relative response to the primary ion over the other ions present in the solution.51 Potentiometric selectivity coefficients were measured by the separate solution method (SSM).37 The values of the selectivity coefficients for electrodes no. 14 and 16 are given in Table 4. For electrode no. 14, all the selectivity values are <1 except for Al3+, while the values of all the interfering cations in case of sensor no. 16 (with 1% MWCNT) are less than one, indicating that these cations have negligible disturbance on the functioning of the investigated electrodes, and the enhancement of the selectivity pattern observed by adding the MWCNT (Table 4).
Table 4 Selectivity coefficient values for the proposed Zn2+-CMCPE
Cation log[thin space (1/6-em)]KPotZn,J
With 1% MWCNT Without MWCNT
Zn2+ 0.00 0.00
Na+ −7.12 −5.00
NH4+ −6.51 −3.72
K+ −7.37 −4.51
Cd2+ −2.23 −1.55
Ba2+ −1.35 −1.62
Ca2+ −1.45 −1.71
Ni2+ −1.25 −1.75
Pb2+ −3.51 −0.34
Mg2+ −1.18 −1.44
Mn2+ −3.15 −7.65
Cu2+ −2.03 0.37
Al3+ −1.35 2.70
Fe2+ −1.32 −0.03
Fe3+ −0.76 0.37

Table 5 The calculated binding energies of several metal ions to the ligand in terms of kJ mol−1
Metal ion Binding energy
Na+ −146.657
Mg2+ −75.6891
Al3+ −291.089
K+ −392.712
Ca2+ −294.171
Mn2+ −65.1576
Ni2+ −19.3597
Cu2+ −44.4163
Zn2+ −77.4744
Cd2+ −454.393
Ba2+ −1039.63
Pb2+ −478.316

3.2. Response mechanism

The mechanism of the investigated Zn2+ sensor was explored by observing the electronic absorption spectra of the naphthalocyanine ionophore in THF before and after the reaction with 0.1 M Zn2+ solution. Porphyrins are characterized by intense and sharp absorption bands in the range of 400–500 nm referred as Soret or B-band. In the range of 500–700 nm, the electronic spectra of porphyrins and their naphthalocyanine derivatives show a group of bands named Q-bands.52 The electronic absorption spectrum of the titled ionophore displayed several bands in the UV-region at 210, 245, 280, 325, and 360 nm as well as a shoulder at 405 nm. Besides, a strong band at 780 nm and two shoulders at 695 and 735 nm are observed in the visible range (Fig. 5). By adding Zn2+ ions to the naphthalocyanine solution, a clean renovation to new species was observed as evidenced by a blue shifted of the Q-band to 760 nm and the disappearance of the band at 360 nm. Another attempt to investigate the mechanism of the electrode towards Zn2+ was obtained using solution FT IR analysis (in THF). The IR spectrum of the free ionophore (Fig. 6(A)) showed a medium band at 1628 cm−1 assigned to ν(C[double bond, length as m-dash]N). This mode was shifted to lower wave number and overlapped with the C[double bond, length as m-dash]C vibrational modes in the same region with the appearance of the band at 1563 cm−1. Therefore, the C[double bond, length as m-dash]N groups are involved in chelation (Fig. 6(B)). Assignment of the NH group before and after adding Zn2+ ions to naphthalocyanine solution is not easy owing to presence of H-bonds between ionophore and THF molecules. For this, the spectrum of the paste containing the ionophore, graphite, FPNPE, and NaTImB was compared to the spectrum of the same paste after keeping in contact with 0.1 M Zn2+ for 1 h and then rinsed with water for 10 s. Comparison between the solid spectra indicated that NH groups are involved in complexation formation as the ν(NH) mode is shifted from 3425 to 3416 cm−1 upon contact with Zn2+. Therefore, naphthalocyanine interacted with Zn2+ as a neutral carrier through four coordination centers.
image file: c5ra12055a-f5.tif
Fig. 5 Electronic spectra of the ionophore in presence and absence of Zn2+ in THF.

image file: c5ra12055a-f6.tif
Fig. 6 FT IR spectra of the modified paste in (A) absence of Zn2+ and (B) presence of Zn2+.

Scanning electron microscope (SEM) is a valuable tool used for characterizing the surface morphology of the sensor. As shown in Fig. 7, the appearance of the metallic zinc on the surface of the sensor having the titled ionophore after its dipping in a solution of Zn2+ for one hour could be taken as evidence for the surface reaction. Energy dispersive X-ray (EDX) is a key tool used for identification of sample, since each element has a unique atomic structure that allow unique set of peaks on its X-ray spectrum. The growing of a sharp and intense peak for zinc at about 8.6 keV in the EDX spectrum of the surface of the electrode after dipping in zinc solution (Fig. 7) is a sign for the successful reaction of the titled ionophore with the Zn2+ ions at the electrode surface.


image file: c5ra12055a-f7.tif
Fig. 7 SEM images for the Zn2+-MWCNTCP before (a1) and after the reaction with zinc ions (b1 and b2) at 5000, 20[thin space (1/6-em)]000 magnification factor respectively and EDX analysis for the same electrode before (a2) and after (b3) the reaction with zinc.

3.3. Quantum chemical calculations

The most essential property for metal ion selectively with naphthalocyanine is the binding energy (B.E.). Binding energies are defined as the total energy of complex minus the sum of total energies of the most stable isolated moieties, i.e., metal ion, and free naphthalocyanine. The energies of the free metal ions and naphthalocyanine have been calculated in the aqueous phase at Hartree–Fock (HF) level of theory. The standard 6-31G(d) basis set was used for Na+, Mg2+, Al3+, K+, Ca2+, Mn2+, Ni2+, Cu2+, and Zn2+, while LANL2DZ was applied for Cd2+, Ba2+ and Pb2+. The calculated values of binding energies are given in Table 5. The binding energies sequences follow the order Ba2+ > Pb2+ > Cd2+ > K+ > Ca2+ > Al3+ > Na+ > Zn2+ > Mg2+ > Mn2+ > Cu2+ > Ni2+. The titled ionophore has a large flexible cavity, which can accommodate a variety of metal ions to form stable coordination compounds. The interference coming from the stable isoelectronic configurations of neon (Na+ and Al3+), argon (K+ and Ca2+), and xenon (Ba2+), Cd2+ ([Kr]4d10), as well as Pb2+ ([Xe]4f145d106s26p2) might be attributed to several explanations. First, it is necessary to clarify that our calculations were performed on a single molecule in the aqueous phase, rather than the experimental measurements which were carried out on the bulk solution. Second, the solvent properties such as hydrogen-bond ability and solute–solvent interactions cannot be provided by PCM model and these variables are well known to affect the selectivity of the titled ionophore, and also the value of formation constant. The stereochemistry of the formed metal complexes may be one of the reasons which did not be considered in the calculations as our assumption was based on the formation of square-planar geometry. In quantum chemical calculations, it is difficult to take into consideration several variables such as the complexity of the paste (additives, plasticizer, etc.) and electrode–solution interface. Finally, the energies values of the free interfering cations were found to be lower than Zn2+ ion, while the corresponding complexes have higher energies than the zinc ionophore complex, despite these ions have little ability towards the formation of metal complexes owing to the absence of incomplete d-orbitals.

Natural bond orbital (NBO) analysis was carried out for the complexes of Mn, Ni, Cu, and Zn to shed more light about the preference of the investigated naphthalocyanine towards Zn among the transition metal ions. NBO can provide details about the type of hybridization, the nature of bonding and strength of the interactions between metal ion and donor sites.41 According to NBO, the electronic configurations of Mn, Ni, and Cu in naphthalocyanine complexes are [Ar]4s0.133d5.204p0.19, [Ar]4s0.253d8.154p0.25, and [Ar]4s0.293d9.754p0.21 comparing with [Ar]4s0.253d9.954p0.27 for the Zn complex. The calculated natural charges were found to be 1.478e, 1.346e, 0.752e, and 1.528e for Mn, Ni, Cu, and Zn in that order. The quantity of electron density donation from the donor sites of the ionophore to the metal ions is not the same. The natural charges are more reduced in case of Ni and Cu comparing with Mn and Zn as the half- and completely filled d-orbitals, which give extra stability to these complexes. The increase in the binding energy of Zn2+ with respect to the other transition metal elements may be assigned to the back donation from the completely filled d-orbitals to the empty p-orbitals in the titled ionophore.

The energies and compositions of the frontier molecular orbitals53 are important properties in several chemical and pharmacological processes. EHOMO is associated with the electron donating ability, while ELUMO indicates the ability of the molecule to accept electrons. The chemical hardness and softness of a molecule as good indicator for the chemical reactivity of a given molecule can be calculated from the energy values of EHOMO and ELUMO. HOMO–LUMO gap (ΔE) decides whether the molecule is hard or soft; a soft molecule is more polarizable than the hard one. Hard molecules need small energy to excitation. Softness (S) is a property of molecule that measures the extent of chemical reactivity. It was found that Mn, Ni, Cu, and Cd complexes are harder than Zn(II) complex. Dipole moment (μ in Debye) was used to qualitatively analyze the trend in the hydrophobic values (log[thin space (1/6-em)]P). A very significant dipole moment may polarize the molecule in such a way that it produces a required potential at several atomic centers necessary for binding and activity. Herein, the higher dipole moment values were reported for Pb, Al, Ca, and Zn in that order. Fig. 8 shows the optimized structures of free ionophore, and zinc-ionophore complex; HOMO and LUMO of zinc-ionophore complex.


image file: c5ra12055a-f8.tif
Fig. 8 Optimized structures of (a) free ionophore, and (b) zinc-ionophore complex; (c) HOMO of zinc-ionophore complex, and (d) LUMO of zinc-ionophore complex.

3.4. Analytical applications

3.4.1. Real samples analyses. The Zn(II)-MWCNT-CPE was successfully used for the determination of Zn2+ ions in the real samples. For example, one gram of Mamy-vit, as a multivitamin drug (APEX pharma S.A.E.), supplied from a mixture of ten capsules was dissolved in 5 mL of concentrated nitric acid and digested to dryness. Then 3 mL more of nitric acid was added to the dry beaker and the pH was adjusted to the desired pH of the investigated sensor, further by dilution with water to 500 mL.54 Another real sample was obtained from Prisoline zinc eye drop (Khaira pharma and chem. InD). The working sample was prepared by diluting a volume of 10 mL to 100 mL with water. Furthermore, different water samples were analyzed coming from tap water, and from granite factory. The pH of these samples was adjusted. The concentration of Zn2+ ions was measured by the proposed Zn(II)-MWCNT-CPE and atomic absorption spectrometry (AAS) for comparison. As shown in Table 6, a superior agreement between the two methods as confirmed by the high recovery values (97.8–102.4%) and the low RSD% values.
Table 6 Determination of zinc in multivitamin, eye drop, and water samples using Zn2+-MWCNT-CPE
  Found (μg mL−1) Recovery% Relative standard deviationa%
Potentiometrically AAS
a Average of four replicates.
Mammy vit 24.60 25.00 98.40 1.33
Eye drop 70.11 70.48 99.47 1.11
Tap water 2.09 2.14 97.80 1.66
Waste water 0.68 0.66 102.40 1.32
  Taken (μg mL−1) Found (μg mL−1) Recovery% Relative standard deviationa%
Pure samples 0.49 0.50 98.00 1.02
0.98 1.00 98.00 1.12
2.99 3.00 99.60 1.04
4.99 5.00 99.80 1.10

3.4.2. Potentiometric titrations. Zn(II) ions can be titrated with potassium ferrocyanide according to the following equation,55
2K4[Fe(CN)6] + 3Zn(NO3)2 = K2Zn3[Fe(CN)6]2 + 6KNO3
where Zn(II)-MWCNT-CMCPE was used as an indicator. In this titration, 10 and 15 mL of 1 × 10−4 mol L−1 Zn2+ and 10, 15, 20 mL of 1 × 10−3 mol L−1 Zn2+ and 10, 15 mL 1 × 10−2 mol L−1 Zn2+ solution were titrated against 1 × 10−4, 1 × 10−3, and 1 × 10−2 mol L−1 ferrocyanide solutions, respectively (Fig. 9). The obtained results revealed that the potential difference of the electrode is increased upon the addition of the ferrocyanide solution with the detection of a sharp end-point. The data (Table 7) shows the accuracy and reproducibility of the electrode.
image file: c5ra12055a-f9.tif
Fig. 9 Potentiometric titration curves of 10, 15 mL 10−4 M Zn2+ with 10−4 M ferrocyanide solution (a and b), and 10, 15, 20 mL 0.001 M Zn with 0.001 M ferrocyanide (c–e) and 10, 15 mL 0.01 M Zn with 0.01 M ferrocyanide (f and g) using Zn2+-MWCNT sensor.
Table 7 Potentiometric titration for determination of zinc in pure solution using Zn2+-MWCNT-CPE
Taken (μg mL−1) Found (μg mL−1) Ra RSDb Jump, mV
a R = recovery.b RSD = relative standard deviation.
6.50 6.51 100.15 1.34 45
9.75 9.65 98.97 1.21 65
65.00 64.74 99.60 1.22 80
97.50 96.40 99.38 1.12 95
130.00 129.90 99.92 1.01 100
650.00 641.50 98.69 1.00 125
975.00 970.10 99.49 1.45 230

3.5. Validation of the proposed electrode

The reproducibility and stability of the electrode were investigated by repeating the preparation and the calibration of each composition four times. As shown in Table 1, the RSD% values were found to be in the range of 0.21–2.56%. The potential readings for the electrode no. 16 dipped in 1.0 × 10−4 mol L−1 Zn2+ solution ten times are negligibly changed (±0.5 mV) over 4 h, which indicates the good reproducibility and high stability of the electrode. The accuracy was expressed in terms of percentage deviation of the measured concentration from the actual concentration. The obtained results (Table 6) are within the acceptance range of 3%.

The ruggedness of the potentiometric method was carried out by constructing the calibration curve using the same working solution, electrode, and experimental conditions over four days, and with four different persons. The RSD% values were found to be less than 1% for the repetitive experiments in four days, and <2% for the repetitive experiments by the different workers. The obtained data revealed that the method is reproducible.

4. Conclusion

2,11,20,29-Tetra-tert-butyl-2,3-naphthalocyanine as a neutral carrier contained in a modified carbon paste electrode has been used for the determination of Zn2+ ions (in the nano-molar levels) in the real samples such as multivitamin drug, Prisoline zinc eye drop and water samples (tap water and granite factory). Addition of a multi-walled carbon nanotube (MWCNT) to this electrode resulted in a great enhanced sensitivity observed by the lower detection limit, 5.0 × 10−9 mol L−1. Besides, this sensor has higher electrical conductivity and higher resistance to interferents. High sensitivity, and selectivity, a wide range, a low detection limit, and very fast response time are the significant properties of the investigated electrodes. The increase in the binding energy of Zn2+ with respect to the other transition metal elements may be assigned to the back donation from the completely filled d-orbitals to the empty p-orbitals in the titled ionophore.

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